This application claims priority to Japanese Patent Application No. 2000-245801, filed Aug. 14, 2000.
1. Field of the Invention
The present invention relates in general to a two-stroke internal combustion engine which is suited for use in a portable power working machine, and in particular to a two-stroke internal combustion engine which is designed to introduce air into a combustion actuating chamber prior to the introduction of the air-fuel mixture, thereby making it possible to minimize the quantity of so-called blow-by or the quantity of unburned air-fuel mixture discharged without being utilized for the combustion.
2. The Prior Art
An ordinary small air-cooled two-stroke internal combustion engine which is conventionally used in a portable power working machine, such as a chain saw and bush cutter, is constructed such that an ignition plug is disposed at the head portion of the cylinder and an intake port, a scavenging port and an exhaust port, which are to be opened and closed by a piston, are provided at the trunk portion of the cylinder. According to this two-stroke internal combustion engine, one cycle of the engine is accomplished by two strokes of the piston without undergoing a stroke which is exclusively assigned to the intake or exhaust.
More specifically, in the ascending stroke of the piston, an air-fuel mixture is introduced from the intake port into the crankcase disposed below the piston. During the subsequent descending stroke, the air-fuel mixture is pre-compressed producing a compressed gas mixture, which is then blown into a combustion actuating chamber which is disposed above the piston, thereby enabling waste combustion gas to be discharged through the exhaust port. In other words, since the scavenging of the waste combustion gas is effected by making use of the gas flow of the air-fuel mixture, the unburned air-fuel mixture is more likely to be mingled into the combustion gas (exhaust gas), thereby increasing the quantity of so-called blow-by or the quantity of air-fuel mixture that is discharged into atmosphere without being utilized for the combustion. Because of this, the two-stroke internal combustion engine is not only inferior in fuel consumption, but is also disadvantageous in that large amounts of poisonous components, such as HC (unburned components in fuel) and CO (incomplete combustion components in fuel), are included in the exhaust gas as compared with a four-stroke engine. Therefore, even if the two-stroke engine is small in capacity, the influence of these poisonous components on the environmental contamination should not be disregarded.
With a view to addressing these problems, there have proposed various kinds of so-called air-preintroduction type two-stroke internal combustion engines, in which an air inlet passageway for introducing external air is attached to a scavenging passageway so as to permit air to be introduced into the combustion actuating chamber in the descending stroke of the piston prior to the introduction of air-fuel mixture. The air thus pre-introduced functions to form an air layer between the waste combustion gas to be discharged and the unburned air-fuel mixture, thereby preventing the unburned air-fuel mixture from being mixed with the waste combustion gas and thus minimizing the quantity of blow-by of air-fuel mixture. See, for example, Japanese Patent Unexamined Publications H9-125966 and H5-33657, and Japanese Patent No. 3040758.
In such conventional air-preintroduction type two-stroke internal combustion engines, however, air inlet passageways (generally, since one or more pairs of right and left scavenging passageways are disposed, the air inlet passageway is bifurcated) are required to be separately installed outside the engine body (cylinder and crankcase) in order to introduce air into the scavenging passageway, etc. Also, a check valve, such as a reed valve, is required to be provided at a downstream end portion of each of air inlet passageways (at an upstream portion of each of one or more pairs of right and left scavenging passageways). Consequently, these conventional engines are accompanied by various problems, including an increased number of parts is required, the peripheral structure around the engine body, including the air-introducing passageways, is complicated, larger in size as a whole, and heavier in weight, the external appearance and assembling properties of the entire structure are poor, and a large space is required for the installation of the engine, which necessitates further improvements in order to mount the engine on a portable power working machine.
Additionally, since the crankcase is generally formed of a two-part structure (vertically split structure), it is troublesome both in working and assembly, and it is also difficult to reduce the weight and cost thereof.
The present invention overcomes the aforementioned problems associated with the conventional air-preintroduction type two-stroke internal combustion engine.
It is an object of the present invention to provide an air-preintroduction type two-stroke internal combustion engine which is capable of reasonably and compactly arranging the peripheral components of the internal combustion engine, thereby making it possible to reduce the number of parts, to lighten the weight thereof, to save the manufacturing cost thereof, and to simplify the working and assembling thereof.
The aforementioned and other objects of the invention are realized by the provision of a two-stroke internal combustion engine which, in accordance with the present invention, includes: a cylinder in which a piston is adapted to be fittingly inserted; a combustion actuating chamber disposed above the piston; a crankcase hermetically and contiguously disposed below the cylinder and defining a crank chamber; a means for forming an air-fuel mixture; one or more pairs of scavenging passageways, each allowing the combustion actuating chamber to be communicated with the crankcase, symmetrically provided on both sides of a longitudinal section which divides an exhaust port provided on the sidewall of the cylinder into two equal parts; the scavenging passageways allowing air to be introduced therein; the air-fuel mixture-forming means allowing the air-fuel mixture to be introduced therefrom into the crank chamber; in the descending stroke of the piston, the exhaust port is first opened before a scavenging port formed at a downstream end of each scavenging passageway is opened; air is introduced via the scavenging passageways into the combustion actuating chamber prior to the introduction of the air-fuel mixture; and an air inlet passageway for introducing air into the scavenging passageways is provided at the mating face between the cylinder and the crankcase.
In one embodiment, the crankcase is provided with an air inlet port for introducing air into the air inlet passageway, and the air inlet port is provided with a check valve. Preferably, the air inlet passageway is an annular passageway formed at and opening downwardly through the lower end portion of the cylinder, the underside of the annular passageway being closed by an upper wall of the crankcase. The crankcase is preferably provided with a communicating passageway which communicates the annular air inlet passageway with the crank chamber, and the air inlet passageway also functions as an upper (or upstream) portion of the scavenging passageway.
In another embodiment, the air inlet port is provided with a pair of inlet ports communicating with the air inlet passageway, and a single reed valve functioning as the check valve is provided for opening or closing the pair of inlet ports.
The crankcase is preferably formed of a horizontally split structure having an upper crankcase and a lower crankcase, wherein the crankcase is divided by a horizontal plane passing through the central axis of a crankshaft which is rotatably supported by the crankcase, thereby forming the upper crankcase and the lower crankcase. Preferably, in this case, the lower crankcase is formed integrally with part of a fan casing for the engine.
In still another embodiment, the internal combustion engine is further provided with an air-feeding passageway for feeding air to the air inlet passageway through the air inlet port, and with an air-fuel mixture-feeding passageway for feeding the air-fuel mixture generated by the air-fuel mixture generating means to the crankcase, the air-feeding passageway and the air-fuel mixture-feeding passageway being arranged side by side.
In another embodiment of the invention, the air-fuel mixture generating means is a carburetor which is provided with the air-feeding passageway and the air-fuel mixture-feeding passageway, each passageway being provided with a throttle valve, and these throttle valves being linked with each other.
According to the preferred embodiments of two-stroke internal combustion engine of the present invention as described above, in the ascending stroke of the piston the external air is sucked via the air-feeding passageway, the air inlet port and the reed valve functioning as an air check valve into the air inlet passageway, and at the same time, this external air is introduced via the air inlet passageway into the scavenging passageways and the crank chamber so as to be reserved therein, and the air-fuel mixture provided by the air-fuel mixture-generating means is sucked via the reed valve functioning as an air-fuel mixture check valve into the crank chamber so as to be reserved therein.
When the air-fuel mixture inside the combustion actuating chamber disposed above the piston is ignited and burned, the piston is caused to be pushed downward due to the generation of combustion gases. In the descending stroke of the piston, the air and the air-fuel mixture existing in the air inlet passageway, the scavenging passageways and the crank chamber are compressed by the piston. At the same time, an exhaust port is opened, and, upon further descent of the piston, the scavenging port provided at a downstream end of each of the scavenging passageways is opened. During this scavenging period, only the air which has been contained in the scavenging passageways and the air inlet passageway and compressed by the piston is permitted to be introduced from the scavenging ports into the combustion actuating chamber.
As the piston further descends, the introduction of air from the scavenging ports to the combustion actuating chamber is completed, after which the air-fuel mixture that has been precompressed in the crank chamber is introduced via the scavenging passageways into the combustion actuating chamber until the scavenging period is completed.
As mentioned above, since air is introduced into the combustion actuating chamber from the scavenging ports prior to the introduction of the air-fuel mixture in the descending stroke of the piston, the waste combustion gas is forced, by this action of air, out of the exhaust port and is then discharged via a muffler into the external atmosphere without leaving a residue of waste combustion gas either in the combustion actuating chamber or in a portion near the inner wall of cylinder which is disposed opposite to the exhaust port.
In this case, a layer of the air that has been introduced in advance from the scavenging ports into the combustion actuating chamber is formed at an interface between the waste combustion gas and the air-fuel mixture that has been introduced later from the scavenging ports into the combustion actuating chamber. Due to the existence of this air layer, the air-fuel mixture is effectively prevented from being mixed with the waste combustion gas, thereby realizing a stratified scavenging. As a result, the quantity of so-called blow-by or the quantity of unburned air-fuel mixture discharged is minimized, thus making it possible to reliably and completely ignite the air-fuel mixture, to improve fuel consumption and to reduce the content of poisonous components in the exhaust gases.
Further, according to the two-stroke internal combustion engine of the present invention, an air inlet passageway for introducing air into the scavenging passageways is provided at the mating face between the cylinder and the crankcase, and in a more preferable embodiment, only a single-piece air check valve is provided. By contrast to the conventional internal combustion engine, where bifurcated air inlet passageways are required to be separately installed outside the engine body (cylinder and crankcase) and, still further, a check valve such as reed valve is required to be provided at a downstream end portion of each air inlet passageway (at an upstream portion of one or more pairs of right and left scavenging passageways), it becomes possible according to the present invention to reasonably and compactly arrange the peripheral components of the engine, thereby making it possible to reduce the number of parts, to lighten the weight thereof, to save the manufacturing cost thereof, and to simplify the working and assembling thereof.
As the crankcase is formed of a horizontally split structure having an upper crankcase and a lower crankcase, wherein the crankcase is divided by a horizontal plane passing through the central axis of a crankshaft, it becomes possible, as compared with the conventional crankcase of the prior art two-part structure (vertically split structure), to simplify the working and assembling of the engine and, at the same time, to increase the ratio of using synthetic resin, to thereby lighten the weight and save the manufacturing cost of the crankcase.
Since the feeding of air is performed using piston pumping and not an external pump, the entire structure of the engine can be simplified and the manufacturing cost thereof can be reduced